Edward C. answered • 03/19/15

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Note that 3 weeks = 21 days

The payday loan charges 0.75% per day for 21 days = 0.75% * 21 = 15.75% total

15.75% of 320 is 0.1575*320 = $50.40

For the credit card you need to use the formula for compound interest which is

A = P*(1 + r/n)

^{n*t }whereA = Accumulated or final amount

P = Principal or original amount (320 in this case)

r = interest rate as a decimal (0.168 in this case)

n = number of compoundings per year (365 in this case)

t = time in years (21/365 in this case)

A = 320*(1 + 0.168/365)

^{365*(21/365) }A = 320*(1 + 0.000460274)

^{21}A = 320*(1.000460274)

^{21}A = 320*1.00971

A = 323.1072 = 323.11 to the nearest cent

This amount includes both principal and interest, so the interest cost is

323.11 - 320 = $3.11